1. Field of the Invention
This invention relates to a semiconductor accelerometer. More particularly, the present invention relates to an accelerometer suitable for control of an automobile, an engine, an air bag, etc, and a production method thereof.
2. Description of the Related Art
One of the requirements imposed on an accelerometer for automobiles is that the accelerometer can accurately detect acceleration of a relatively low level (0 to .+-.1 G) at a low level frequency (0 to 100 Hz). Incidentally, the term "1 G" used herein means a unit of acceleration and represents 9.8 m/sec.sup.2.
A piezoelectric type sensor utilizing the piezoelectric effect, a magnetic type sensor utilizing a differential transformer, a semiconductor strain gauge type sensor and an electrostatic capacitance type sensor utilizing fine silicon etching technology, and so forth, are all well known as such accelerometers. Among them, the semiconductor type accelerometer is believed to be most promising because it can accurately detect a low level acceleration at a low frequency level, is economical, and is suitable for mass-production.
The electrostatic capacitance type sensor is characterized in that its sensitivity is higher than that of the strain gauge type.
FIG. 66 of the accompanying drawings illustrates an electrostatic capacitance type accelerometer as a prior art example disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-134570. In the electrostatic accelerometer shown in FIG. 66, a detection portion of the sensor is formed by directly bonding three silicon substrates 300, 301 and 302 through thermal oxide films 303 as insulating films and coupling them together. A silicon beam (beam-like portion) 304 and a movable electrode 305 are in advance formed on the silicon substrate 300 by an etching process before bonding. Fixed electrodes 306 and 307 made of poly-Si (silicon) are in advance formed on the silicon substrates 301 and 302, respectively. A movable electrode 305 having the function of a weight is supported by a silicon beam 304, and the dimension of a gap between the movable electrode 305 and the fixed electrodes 306, 307 changes in accordance with magnitude of acceleration acting on the movable electrode 305 in a vertical direction of the drawing. In other words, the electrostatic capacitance of the gap portion changes in accordance with acceleration acting on the detection portion, and acceleration can be detected by transmitting this change to an external electronic circuit through a bonding pad 308.
In the electrostatic type accelerometer having a structure described above, however, a high level of fabrication technique is necessary so as to etch the silicon substrate to an accuracy of 100 to 200 .mu.m so as to form the beam and the production cost thus increases.
In other words, one silicon substrate for forming the movable electrode and two silicon substrates for forming the fixed electrodes, that is, three substrates in all, are necessary, and a reduction in cost is difficult. Furthermore, since the silicon substrates must be bonded to one another through the thermal oxide film, thermal limitations are imposed on the fabrication process. Because detection of acceleration relies on the change of the electrostatic capacitance, the electrode area for forming the electrostatic capacitance cannot be reduced below the lower limit for measurement, and the sensor cannot be made compact in size.